Traditional timers use a combination of Timer and TimerTask
java.util.Timer: A timer, which is used to execute a task once after a specified time, or to repeat it periodically.
java.util.TimerTask: inherits the Runnable class, which is essentially a Runnable object
There are two main usage scenarios for Timer and TimerTask timers:
1) After how long, execute the timer task once
2) After how long, execute a timer task, and then execute a task at the same time interval
First, the use of timer
package com.sam;
import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;
/**
* Review of traditional timers
* @author SAM
*
*/
public class TraditionalTimerTask {
/**
* @param args
*/
public static void main(String[] args) {
Timer timer = new Timer ();
timer.scheduleAtFixedRate(new TimerTask() {
@Override
public void run() {
System.out.println("定时器 TName=" + Thread.currentThread().getName() + ",bombing!");
}
}, 1000L, 3000L);
// For observation, print the current time
while (true){
System.out.println(new Date().toLocaleString());
try {
Thread.sleep(1000L);
} catch (InterruptedException e) {
e.printStackTrace ();
}
}
}
}
Results of the:
2017-5-13 17:23:30
Timer TName=Timer-0, bombing!
2017-5-13 17:23:32
2017-5-13 17:23:33
2017-5-13 17:23:34
Timer TName=Timer-0, bombing!
2017-5-13 17:23:35
2017-5-13 17:23:36
2017-5-13 17:23:37
Timer TName=Timer-0, bombing!
2017-5-13 17:23:38
2017-5-13 17:23:39
2017-5-13 17:23:40
Timer TName=Timer-0, bombing!
It can be seen that the timer is executed after 1 second for the first time, and the timer task is repeated every 3 seconds after that.
Second, the timer consolidation
Requirement: execute a task after 2 seconds, then execute a task after 4 seconds, and then return to execute a task after 2 seconds,
Then execute the task again in 4 seconds, and so on, please write the code.
package com.sam;
import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;
/**
* Review of traditional timers
* Requirement: execute a task after 2 seconds, then execute a task after 4 seconds, and then return to execute a task after 2 seconds,
* Execute a task again in 4 seconds, and so on, please write the code.
* @author SAM
*
*/
public class TraditionalTimerTask_2 {
/**
* @param args
*/
public static void main(String[] args) {
// Execute the scheduled task after 2 seconds
new Timer().schedule(new MyTimerTask(), 2000L);
// For observation, print the current time
while (true){
System.out.println(new Date().toLocaleString());
try {
Thread.sleep(1000L);
} catch (InterruptedException e) {
e.printStackTrace ();
}
}
}
}
class MyTimerTask extends TimerTask{
static int count;
@Override
public void run() {
// 4 seconds for odd numbers, 2 seconds for even numbers
count = ++count % 2;
System.out.println("定时器 TName=" + Thread.currentThread().getName() + ",bombing!");
new Timer().schedule(new MyTimerTask(), 2000L + 2000L*count);
}
}
Results of the:
2017-5-13 17:45:52
2017-5-13 17:45:53
Timer TName=Timer-0, bombing!
2017-5-13 17:45:54
2017-5-13 17:45:55
2017-5-13 17:45:56
2017-5-13 17:45:57
Timer TName=Timer-1, bombing!
2017-5-13 17:45:58
2017-5-13 17:45:59
Timer TName=Timer-2, bombing!
2017-5-13 17:46:00
2017-5-13 17:46:01
2017-5-13 17:46:02
2017-5-13 17:46:03
Timer TName=Timer-3, bombing!
Then go into some details of the Timer class. Timer has two sets of methods for starting timer tasks, which may be confusing:
1）schedule(TimerTask task, long delay, long period)
2）scheduleAtFixedRate(TimerTask task, long delay, long period)
the difference:
他们都可以实现：多长时间后，执行一次定时器任务，之后每间隔相同时间执行一次任务；
但是他们又有一些本质的区别，首先schedule是固定延期执行，而scheduleAtFixedRate是固定速率执行。
再具体说：
1）固定延期，根据前一次执行的实际时间来安排每一次执行。
如果由于任何原因（如垃圾回收或者其他后台活动）而延迟了某次执行，则后续执行也将延迟。
2）固定速率，根据已安排的初始执行时间来安排每次执行。
如果由于任何原因（如垃圾回收或者其他后台活动）而延迟了某次执行，
则将快速连续执行两次或者更多任务，从而使后续执行能够“追赶上来”。
注意这时TimerTask的执行需要考虑同步。
最后推荐一个开源项目，quartz，主要用来实现复杂的调度需求。
例如周一至周五早上10点定时收邮件，周六日不收邮件，
这时使用Timer和TimerTask可能比较复杂，使用quartz开源项目可以很容易的解决这类需求。